Editors’ Highlights

Tides and Waves Interact to Cause Hurricanes in Near-Space

The interaction of tides and waves generated in the lower atmosphere can cause the mean zonal wind speed in the lower ionosphere to oscillate equivalent to a category 1 hurricane at Earth’s surface.

Source:
Journal of Geophysical Research: Space Physics

The diurnal-mean amplitudes of zonal wind over the equator at 97 km altitude as a function of longitude and day of month during October 2009 (days of the month on x axis) as determined by forcing the general circulation model with observational data from lower altitudes. Note the amplitude variations of the zonal wind and its periodic nature. Credit: Forbes et al., 2018, Figure 3a

Although considerable progress has been made recently regarding vertical coupling in the atmosphere, the mechanisms of planetary wave and tidal coupling to the thermosphere-ionosphere system is not well understood. A critical reason is that the amplification and dissipation processes of waves and tides are highly dependent on the wave sources, sinks and propagation conditions including the winds in the thermosphere. Forbes et al. [2018] explore this topic with a series of numerical experiments using a sophisticate thermosphere-ionosphere global circulation model. They show that the dissipation of the tidal spectrum modulated by planetary waves causes the thermosphere to oscillate by tens of meters per second over a range of planetary wave periods (typically 2-16 days). Also, the same tidal spectrum can amplify the penetration of westward-propagating planetary waves in the dynamo region, through nonlinear wave-wave interactions. The study provides a new framework for other researchers to explore these complex interactions and has implications for those interested in satellite drag and radio propagation as some of the effects reach F-region altitudes (~350 km).

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